In recent years, an aluminum alloying process has been applied to parts of vehicles, industrial instruments, and the like. Because a usage environment of such parts is severe, anodizing is applied in view of abrasion resistance and high strength. JP3129494B (hereinafter referred to as Reference 1) discloses a piston for an internal combustion engine where an anodic oxide coating is formed on a surface of a piston base material. According to the piston disclosed in Reference 1, silicon grains are removed from a lower surface of a land groove formed at a land portion of the piston. Then, the anodic oxide coating is applied to the land groove where the silicon grains are removed. In addition, JP08-209389 (hereinafter referred to as Reference 2) discloses a technology for forming an anodic oxide coating on a wall surface of a ring groove of a piston. The hardness of the anodic oxide coating is generally in a range from HV (Vickers Hardness) 200 to HV 400.
Further, an electrolytic oxidation that is also called a plasma electrolytic oxidation and that includes a more prominent coating than the anodic oxide coating for the abrasion resistance, the high strength and a surface roughness has been attracting a lot of attention. In the electrolytic oxidation, because a surface of an aluminum member is formed by a hard electrolytic oxidation ceramic coating mainly constituted by an alpha alumina, the aluminum member is given prominent characteristics in view of the abrasion resistance, the high strength and the surface roughness.
WO2005-118919 (hereinafter referred to as Reference 3) discloses an electrolytic oxidation that is also called a plasma electrolytic oxidation. According to the electrolytic oxidation disclosed, in a state where a processed part is immersed in an alkaline electrolyte in which a zirconium compound is included, an electrolytic oxidation ceramic coating that includes a metal element of a base material element and a zirconium is formed at the processed part by use of an alternating current voltage. The electrolytic oxidation ceramic coating has the hardness of HV 800 or more because of a dispersed phase of a microcrystal of a dispersed zirconium oxide.
According to the electrolytic oxidation ceramic coating formed by the technology disclosed in Reference 3, a large surface projection may be generated at a surface layer, which leads to a rough surface. Thus, an abrasion tends to originate from the surface projection, which results in a large abrasion amount of the coating itself and a high aggressiveness to the other member such as a mating member caused by abrasion powder, and the like. In particular, in a case where silicon is included in a base material of the aluminum alloy, a silicon oxide is generated on the silicon and thereon further laminated is a zirconium oxide. As a result, a large surface projection tends to be generated at the electrolytic oxidation ceramic coating. When the electrolytic oxidation ceramic coating slides with the mating member, the abrasion tends to originate from the surface projection, which leads to the large abrasion amount of the coating itself and the high aggressiveness to the mating member as mentioned above.
A need thus exists for an aluminum alloy member and a method for manufacturing the same which is not susceptible to the drawback mentioned above.